Such an apparatus is disclosed in U.S. Pat. No. 2,877,548. In this prior-art apparatus the longitudinal bounding walls of a hair-entry aperture are inclined from the outside towards the inside in a direction opposite to the driving direction. The object of this is to provide an improved support and positioning of a hair to be cut by the external cutting member. Moreover, this yields a sharp cutting edge at the inner side of the external cutting member and the facing bounding wall also terminates in a sharp edge at the outer side of the external cutting member, which enhances the erection of hairs which lie flat against the skin.
It has been found that this prior-art construction performs less satisfactorily, which may be explained by the fact that the inclined bounding surface supporting the hair during cutting exerts an outwardly directed force on the hair or the skin area surrounding the hair. This force pushes the hair outwards so that it is not severed over an optimum length.
The hair-entry apertures employed in practice almost exclusively have bounding walls oriented transversely of the driving direction, as shown in FIG. 1 of said U.S. Pat. No. 2,877,548.